Introduction

Dual-purpose (DP) bovine production, in tropical regions of Mexico and Latin America, rely on the use of local resources like grasses, shrubs and trees under extensive management. In the south west of the State of Mexico as well as in most tropical regions of México, cattle feed exclusively on forages under extensive grazing during the rainy season. During the dry season (December to May), the availability and nutritional quality of forages decreases considerably. To minimize the impact of the low forage availability and to the diminished quality of forages, farmers supplement their cattle with variable amounts of supplement (5 to 9 kg DM/cow/d)¹.

Farmers’ decision on the amount of supplement offered to each cow, and the time to start supplementation during the dry season, depends on the availability of forage on pastures. The second half of the dry period (March to May) is the most critical for farmers, since forage in pastures becomes scarce, so that farmers use supplements to sustain animal production^1,2.

Metabolizable energy has been reported as one of the main constrains for cattle production under tropical conditions due to the low nutritional value of forages (due mainly to high fibre content)³.

Supplements represent between 50 to 70 % of milk and beef production costs. Due to supplementation, milk production costs increase 22 % in the dry season, in comparison to the rainy season, reducing the already slim profit margins^1,2.

In order to keep supplement costs as low as possible, harvested maize ears produced in the farm are cracked instead of being ground, in order to reduce processing cost. However, total digestibility of cracked maize ears is lower (87.6 %), compared with ground maize (91.7 %)⁴. Frequently, un-degraded large particles of maize appear in faeces representing a waste and inefficient use of this resource.

Maize starch is the most common source of energy for dairy cattle, that degrades between 4 to 6.4 %/h. Carbohydrate sources with faster degradation rates than maize may improve ruminal conditions, resulting in better animal productive response to supplementation⁵.

Sugar cane molasses is a readily source of energy, that has been used in supplements for cattle feeding on low quality grasses in tropical regions^6,7,8. However, despite availability and relatively low cost, farmers in the study region do not incorporate this resource in their cattle supplements.

The inclusion of sugar cane molasses under in vitro studies improves fibre digestibility of a combination of star grass (Cynodon nlemfuensis) and Leucaena leucocephala; whereas the inclusion of maize grain increased in vitro volatile fatty acids production⁹. Furthermore, addition of sugar cane molasses to supplements based on maize silage, improved growing rates of heifers under tropical conditions, reducing production costs at the same time⁶.

The aim of this experiment was to evaluate the productive and economic response of partial replacement of cracked maize ears (control supplement) with ground maize (20 % inclusion) (GM), or sugar cane molasses (18 % inclusion) (SCM), in supplements offered to grazing dual purpose Brow Swiss cows during the dry season in a subtropical region of Mexico.

Material and methods

Results

Table 2 shows the chemical composition of pasture herbage as well as in vitro gas production parameters. Crude protein average was 58 g/kg DM, having maximum values in wk3 and wk4 (70 and 75 g/kg DM, respectively). Dry matter digestibility (DMd) and estimated metabolizable energy (ME) had the highest values in wk 4 and wk 5 (620 and 606 g/kg DM, and 9.6 and 9.4 MJ/kg DM, respectively).

The asymptotic gas production (b) (mL/g DM), had the highest values in wk 4 and wk 5. The rate of gas production (c) showed the highest values in wk 6 (0.034) and wk 7 (0.039), whereas from wk 1 to wk 5 the rate remained constant ~ 0.032/h. Initial lag time before gas production begins (L) had the lowest value in wk1 (4.4), while from wk 2 to wk 7 values remained close to 5.2.

Average NHA was 11 (kg/ha/d), whereas AHM was 1,932 (kg/ha DM). Pasture morphological composition is shown in Figure 1. Green pasture represented 58 % of AHM, with and increasing trend towards the end of the study, due to some light rains; whereas leaf represented 38 % of the AHM. Cynodon plectostachyus was the predominant grass representing 92 % of the botanical composition; while Paspalum notatum and Paspalum convexum represented 5 and 3 %, respectively.

Figure 1 Available herbage mass (AHM) (kg/ha DM), morphological composition (kg/ha DM) throughout experimental weeks 

Table 1 shows ingredients and chemical composition of supplements. Average DM was 864 g/kg. Crude protein contents were 124 (CS), 113 (GM) and, 119 (SCM) g/kg DM. Neutral detergent fibre of CS was 43 % higher (379 g/kg DM) than experimental supplements (218 and 214 g/kg DM for GM and SCM, respectively).

Molasses inclusion increased values for dry mater digestibility (DMd), organic matter digestibility (OMd) and neutral detergent fibre digestibility (NDFd), as well as estimated metabolizable energy (MJ/kg DM), compared with CS and GM. SCM water soluble content represented by the a fraction produced higher gas volume (68.1), than CS (59.8) and GM (46.5) (Table 1). Soluble fermentation rate of CS was lower (0.098), than GM and SCM (0.128 and 0.153, respectively). Insolubles (b), which is the insoluble but potentially fermentable material was higher for GM (274.2), than CS (256.8) and SCM (255.7). The lag phase was shorter for MOS (4.2 h), intermediate for GM (5.5 h) and longer for CS (5.8 h).

Table 3 shows animal productive response variables. There were no significant differences due to treatments, with the exception of DMI and milk yield (P<0.01). The effect of week was highly significant (P<0.01) for all variables.

Dry matter intake (kg/cow/d) of CS was statistically not different (P>0.05) from SCM (12.2 and 11.7 kg/d, respectively), but significantly different (P<0.05) from GM (11.1 kg/d); whereas GM and SCM were not different from each other (P>0.05).

Milk yield was statistically similar between CS and SCM with 6.2 and 7 kg/cow/d; whereas GM (5.7 kg) was different from SCM but similar to CS. There were no differences (P>0.05) for the rest of the response variables. Fat, protein and lactose mean contents were 33.9, 30.5 and 42.7 (g/kg), respectively. Mean milk urea nitrogen (MUN) was 7.7 (mg/dL).

Live-weight was not different between treatments (430, 406 and 430 kg/cow, for CS, GM and SCM, respectively). Cows given CS, GM and SCM had similar (P>0.05) daily weight gains of 0.283, 0.136 and 0.281 (kg/d), respectively. The average body condition score (BCS) was 1.5 points. Calves mean weight gain was 0.7 (kg/d) (Table 3).

Table 4 shows the partial budget analysis of milk and beef (weaned calves) due to supplements. Molasses supplement had a higher production cost (i.e. total supplement cost), but had better economic returns (i.e. total milk profit margin).

Beef production (kg/treatment as weaned calves) for SCM was higher (399 kg) than CS and GM (371 and 388 kg, respectively); resulting in higher beef incomes and profit margins. GM was second best for both indicators.

Overall, SCM was the treatment with higher total net profit margin from milk plus beef with $1,874 (P<0.01); whereas CS came second ($1,678) and, GM generated the lowest total net profit margin with $1,625.

Discussion

The AHM in grazing areas remained low but constant in the grazing areas. The low but constant forage production during the experiment in spite of dry conditions could be due to water filtered to pastures from a stream that runs across the study area. This may explain in part the constant green material in grazing areas from wk1 to wk9; whereas the sharp increment was due to unusual rain at the end of the study.

In spite of these, the chemical composition of pasture across the experiment was low in terms of CP, DMd, and estimated ME. Similar chemical and agronomic characteristics of pastures dominated by Cynodon plectostachyus, from a nearby location to this study have been reported^17,18.

Molasses inclusion improved in vitro degradability of the supplement given by the fractions a and b, resulting in 0.5 MJ of estimated ME more than CS and GM. This improvement could have had a positive impact on forage digestibility, by improving the ruminal environment due to the supply of readily available energy, which could have increased dry matter intake (additive effect), as demonstrated in previous studies^19,20,21.

The second best supplement was CS, according to soluble fraction a. Better degradation kinetics were expected in the GM than in the CS, since a small particle size of maize grain increases starch digestibility (high soluble fraction)²². However, CS had a higher soluble (a) fraction, higher insoluble (potentially degradable “b”) and higher insoluble fermentation rate, than GM. These could be due to higher proportion of husk and cob material in CS, which have a greater potential degradability, compared with GM.

The low milk production response of cows on GM was unexpected, since ground maize has been reported to yield more energy in rumen in the form of propionate production. Rumen propionate production has been reported as the main driver of milk in lactating dairy cows⁴.

One possible explanation to the low milk production response could be related to the fact that GM had about 20 % less soybean than the other two supplements. Low rumen degradable protein has been related to lower NDF digestibility^23,24.

Furthermore, under low grass quality conditions like in this experiment, sugar cane molasses supplemented with urea, could be a better alternative than ground maize as a source of energy, since sugars are more rapidly fermented in the rumen than the starch from maize, allowing a readily supply of energy to rumen microbes. This may explain the higher soluble fraction and shorter lag phase of SCM²⁵.

Milk yields and milk composition in this study were lower than yields of Holstein and Brown Swiss x Zebu cows^26,27. However, cows in both studies lost weight (~ 40 kg) and BCS, attributed to insufficient nutrients provided by the supplements; contrary to weight gained by the cows in this experiment (~ 0.233 kg/d).

In this study, treatments did not affect milk composition (i.e. fat, protein or lactose); contrary to this, reports show significant differences in protein yield (kg/d) due to a reduction in maize grain particle size that increased starch fermentation, resulting on higher propionate concentration in rumen²³.

Despite cows did gain weight due to supplements (average 0.233 kg/d), body condition score remained unchanged throughout the experiment (~1.5). Dual purpose cows under typical management do not receive enough energy supplies, resulting in small cow size, and limited dry matter intake capacity limiting milk yields. To overcome this situation, it has been proposed supplementation of good quality tropical grasses (0.6 to 4.4 kg/d), and supplements (between 4.0 and 5.0 kg/d), all year round. By doing this, cows will likely be on a better body condition score, (positive energy balance), particularly during critical periods like early lactation, resulting on higher milk yields²⁸.

It is important to note that the weather conditions during the dry season were atypical so the economic analysis should be taken with caution. From the economic point of view, molasses inclusion in the supplement increased profits from milk and beef. The small milk yield difference between these two treatments made a significant economic difference in milk and beef profit margin²⁹.

Combined net profit margins from milk and beef (weaned calves) were around 9 % higher for SCM compared to CS and GM. Molasses has been reported as an energy supplement that results in better milk and beef revenues^29,30. However, these effects do not always happen. In situations when molasses is of high cost, its inclusion in dairy cow supplements represented a loss of revenue due to the small milk response³¹.

Farms in the study region cannot adopt any kind of forage conservation due to the steep conditions of pastures, besides the increased cost due to labour and machinery. Therefore, molasses inclusion in supplements during the dry season could be a supplementation alternative to sustain animal production performance, when forages are limited and of low quality.

Conclusions and implications

Partial replacement of cracked maize ears with sugar cane molasses, in supplements for grazing Brown Swiss dual purpose cows during the dry season, significantly increased milk yields over a supplement with ground maize. There were no differences in other animal productive response variables. Combined net profit margins (milk and calf sales) were on average 9 % higher when including sugar cane molasses in supplements.